add deprecation notice to README (#125)

* add deprecation notice to README

* Update README.md

Co-authored-by: Prithvi Shahi <shahi.prithvi@gmail.com>

---------

Co-authored-by: Prithvi Shahi <shahi.prithvi@gmail.com>

README.md

@@ -1,4 +1,11 @@
-# go-mplex
+# DEPRECATED: go-mplex
+
+mplex is deprecated. When on TCP, we recommend using [Yamux](https://github.com/libp2p/go-yamux/) instead.
+If TCP is not a requirement, consider using QUIC instead.
+
+Ref: Discussion around mplex deprecation: https://github.com/libp2p/specs/issues/553
+
+---
 
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 [![Go Reference](https://pkg.go.dev/badge/github.com/libp2p/go-mplex.svg)](https://pkg.go.dev/github.com/libp2p/go-mplex)